1. Field of the Invention
The present invention relates to an oscillometric type electronic sphygmomanometer which determines (measures) the blood pressure by the steps of compressing an artery with an inflated cuff, detecting pulse wave amplitude while the inflated cuff being deflated and comparing the pressure provided by the cuff on the artery with the pulse wave amplitude.
2. Description of Related Art
When an oscillometric type electronic sphygmomanometer is used, the blood pressure can be measured by the steps of wrapping a cuff around the brachium of a testee or examinee, who may be a patient, for example, compressing the brachium and the like so as for an artery to be occluded by raising the pressure in the cuff, detecting the pressure in the cuff by the means of detecting cuff pressure as the inflated cuff being deflated, detecting pulse wave components superimposed on the signal indicative of the pressure in the cuff (cuff pressure) by the means of detecting pulse wave amplitude, converting the pulse wave information of the pulse wave components into a series of pulse wave amplitudes by the means of detecting pulse wave amplitude, and determining the blood pressure of the testee on the basis of the maximum value of the series of pulse wave amplitudes and the cuff pressure. For example, the cuff pressure corresponding to the maximum pulse wave amplitude is determined as the mean the blood pressure, the cuff pressure corresponding to the pulse wave amplitude of the high pressure side which is equivalent to 50% of the maximum pulse wave amplitude as the systolic blood pressure, the cuff pressure corresponding to the pulse wave amplitude of the low pressure side which is equivalent to 70% of the maximum pulse wave amplitude as the diastolic blood pressure.
During the above process, the pulse wave amplitude gradually increases and then decreases as the cuff pressure changes, thereby forming a peak (a mountain-shaped curve). Normally the pulse wave amplitude changes by first increasing and then decreasing so that only one convex or peak is formed from the beginning to the end of decrease in the cuff pressure, which is as shown in FIG. 4. It is clinically known, however, that the change of the pulse wave amplitude may result in two peaks being formed as shown in FIG. 5 because of the disordered breathing of the testee or the physical movement of his/her body in measuring the blood pressure, or in the case of patients with high blood pressure.
In such cases, a conventional sphygmomanometer may determine the mean, the systolic and the diastolic blood pressure on the basis of the waveform of the pulse wave amplitude which forms the first peak the smaller of the two peaks, thereby generating a measurement containing an error.
An electronic sphygmomanometer has been proposed in Japanese Patent Publication (Kokoku) Hei6-28638 in order to overcome such a problem.
Although the curve passes through a possible maximum value of the pulse wave amplitude and begins to decrease, if the decrement from the possible maximum value is small, the electronic sphygmomanometer described in the Japanese Patent Publication above will not yet determine the true maximum value.
More particularly, it is determined whether A(n).ltoreq..gamma..times.Amax holds and if this inequality does not hold, then the possible maximum value will not be regarded as being the true maximum value by the sphygmomanometer and the measurement will be continued. In the above inequality, A(n) is the measured pulse wave amplitude, Amax is the possible maximum value so far reached, and .gamma. is a predetermined multiplier which is multiplied to Amax and is in the range of 0.7-0.9,for example.
This sphygmomanometer of Japanese Patent Publication (Kokoku) Hei6-28638, however, suffers from the problem below. When the changes in the pulse wave amplitude result in forming two peaks in the entire curve and the decrement of the pulse wave amplitude which forms the first peak is large (for example, when A(n)&gt;0.7-0.9.times.Amax), the maximum value (therefore, the systolic blood pressure and the diastolic blood pressure) will be determined on the basis of the first peak, thereby generating an erroneous the blood pressure value.